Semiconductor strain sensitive element of predetermined temperature coefficient of resistance and method of making same

ABSTRACT

A semiconductor strain sensitive element made by doping impurities into a semiconductor crystal wafer, or strip, to form plural regions of the same conductive type, but different temperature coefficients of resistance, and varying the proportionate volumes of said regions by polishing, or etching, to obtain an element having a predetermined temperature coefficient of resistance.

United States Patent [1 1 Ishii [451 Aug. 7, 1973 1 1 SEMICONDUCTOR STRAIN SENSITIVE ELEMENT OF PREDETERMINED TEMPERATURE COEFFICIENT OF RESISTANCE AND METHOD OF MAKING SAME [75] Inventor:

Katsuyuki Ishii, Nagoya, Japan Kabushiki Kaisha Toyota Chuo Kenkyusho, Nagoya-shi, Japan Filed: Aug. 4, 1970 Appl. No.: 60,830

Assignee:

{30] Foreign Application Priority Data Aug. 7, 1969 Japan 44/62575 [52] US. Cl 29/580, 29/610 SG, 148/185, 148/187 Int. Cl BOIj 17/00 Field of Search 29/580, 583, 576, 29/610 SG; 148/185, 187

[56] References Cited UNITED STATES PATENTS 2,947,924 8/1960 Pardue 29/580 3,406,050 10/1968 Shortes 29/580 3,490,140 l/l970 Knight et a1. 29/576 Primary Examiner-Charles W. Lanham Assistant Examiner-W. Tupman Attorney-Berman, Davidson and Berman [57] ABSTRACT 2 Claims, 13 Drawing Figures PATENTEDAUB H915 3.750.210

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ATTORNEYS.

SEMICONDUCTOR STRAIN SENSITIVE ELEMENT OF PREDETERMINED TEMPERATURE COEFFICIENT OF RESISTANCE AND METHOD OF MAKING SAME BACKGROUND OF THE INVENTION The present invention relates to a semiconductor strain sensitive element having a predetermined temperature coefficient of resistance and to a method of producing the same.

It is an essential requirement of semiconductor strain gauges (hereinafter called strain gauges") that their physical characteristics vary little with temperature, at least in the range of temperature normally encountered during actual use and operation. It is particularly important that their resistance variations with temperature be small in order to prevent serious errors of measurement.

Heretofore, in order to compensate the error caused by change of resistance with temperature, electric circuits have been used embodying plural strain gauges of equal temperature coefficients of resistance and so positioned that the measurement error of one gauge cancels the error of the other. But it has been difficult to make, or select strain gauges having equal, or equal and opposite temperature coefficients of resistance, and it has been inconvenient to actually use them, when found.

Many studies have been conducted and many at tempts have been made to fabricate easily strain gauges having small and preselected coefficients of resistance, without successful results.

The conventional method of producing strain sensitive gauges includes forming semiconductor crystals by the pulling method, or the like, slicing wafers from the crystals, cutting the wafers so as to obtain thin strip elements of desired size for the strain gauges, and connecting leads to the elements. The temperature coefficient of resistance of a semiconductor principally depends upon the crystal condition of the semiconductor, and upon the kind and density of impurities doped into said semiconductor. Therefore, the principal characteristics of conventional gauges are determined during the step of producing the properly doped crystal and, therefore, it has been very difficult to try to adjust the temperature coefficient of resistance by subsequent treatment steps. A very high level technique is necessary to control accurately both the crystal condition of the semiconductor and the density of the impurities. Therefore, it has been very difficult to obtain strain gauges, by the conventional method, which are accurate and have a desired temperature coefficient of resistance even when exercising the most advanced techniques and a great deal of care and attention.

SUMMARY OF THE INVENTION According to the present invention, impurities are partially doped into a semiconductor crystal after it is formed and sliced into a wafer. The wafer, or a thin strip cut therefrom as a base, is so doped as to provide plural regions of a single conductive type (P-type or N- type) and of different coefficients of resistance. A strain gauge of desired temperature coefficient of resistance can thus be accurately produced with ease by adjusting the relative volumes of the said plural regions as, for example, by polishing, or etching off portions of the surface areas of the regions. In this way, the various defects, difficulties, and disadvantages of the conventional method of making a strain gauge are overcome.

To form the plural regions of P-conductive type into a wafer base of N-conductive type, and contrarily to form the plural regions of N-conductive type into a wafer of P-conductive type, are both outside of the present invention. Therefore, the obtained thin element for use as a strain gauge is wholly of a single conductive type and includes plural regions of different temperature coefficients of resistance.

It will be apparent from the above that the primary object of the present invention is to provide a semiconductor strain sensitive element having a predetermined temperature coefficient of resistance and a method of producing the same.

Another object of the present invention is to provide a method of producing a semiconductor strain sensitive element including plural regions of the same conductive type and of different temperature coefficients of resistance by doping impurities into a semiconductor crystal wafer, and adjusting the proportionate volume of said regions to obtain an element having a predetermined temperature coefficient of resistance.

A further object of the invention is to provide a method of producing a semiconductor strain sensitive element having a plurality of doped regions and eliminating a portion of at least one of said regions by polishing, or etching to vary the proportionate region volumes and thereby achieve a preselected temperature coefiicient of resistance with ease and accuracy.

A still further object of the invention is to provide a method for making a semiconductor strain sensitive element which is highly sensitive and has a large output voltage and which is subject to small measurement error due to temperature variation.

Still another object of the invention is to provide a method for producing a semiconductor strain sensitive element having a small temperature coefficient of resistance, high resistivity and little sensitivity variation with temperature.

Yet a further object of the invention is to provide a semiconductor strain sensitive element having a positive, zero, or negative temperature coefficient of resistance, high resistivity, and little variation in temperature within an applied temperature range.

BRIEF DESCRIPTION OF THE DRAWINGS The novel features that are considered characteristic of the invention are set forth with particularity in the appended claims. The invention, itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of specific embodiments when read in connection with the accompanying drawings, wherein like reference characters indicate like parts throughout the several Figures, and in which:

FIGS. 1 to 4 illustrate a first embodiment of the invention by sectional diagrams indicating the sequential steps of production of a strain sensitive element, in which: FIG. I is a sectional view of a semiconductor crystal wafer; FIG. 2 is a similar view of the wafer including a diffusion layer; FIG. 3 is a similar view of a double-layered wafer; and FIG. 4 is a sectional view of a complete strain gauge;

FIGS. 5 and 6 illustrate a second embodiment of the inventive method, in which: FIG. 5 is a sectional view 3 4 of a wafer evaporated with Au after reaching the stage During the steps of polishing and etching mentioned shown in FIG. 2 of treatment as in the first embodiabove, wax is painted on those surfaces which do not ment; FIG. 6 shows the FIG. wafer doped with Au; require treatment so as to prevent such surfaces from FIGS. 7 to 9am sections of strain gauges showing breaking and etching. various modifications of the completed gauges; 5 The strain gauge 40, as shown in FIG. 4, is obtained FIG. 10 is a graph showing the relation between temby cutting the wafer 30 to form an element, or strip, perature and resistance variation for strain gauges 5mm in length and 0.4mm in width, by eliminating the made as in the first embodiment; oxide film on its surface by etching, as described above, FIG. 11 is a graph showing the relation between temand by then connecting leads with ohmic contacts to perature and strain sensitivity variation for strain 10 both ends. As the crystaldirection connecting the elecgauges as made in the first embodiment; trodes of the strain gauge, the 1 I l direction is FIG. 12 is a graph showing the relation between temchosen. perature and resistance variation for strain gauges The resulting strain gauge 40 is constituted as a thin made according to the second embodiment of the instrip including the region I, whose composition is the vcntive method; and 15 same as that of the base wafer 10, and the region 2 is FIG. 13 is a graph showing the relation between temformed by the boron diffusion layer, ohmic metal elecperature and strain sensitivity variation for strain trodes 8 formed by evaporating aluminum on the upper gauges made according to the second inventive method surface of the strip at both ends, and Au leads 9 each embodiment. having one end connected ohmically to an electrode 8.

- Five different strain gauges 40, as the samples A, B, DESCRIPTION OF PREFERRED EMBODIMENTS C, D and E, in Table 1 below, were made using five wa- Embodiment I: In this embodiment of the inventive fers wherein the proportionate volumes of the region 1 method, illustrated in FIGS. 1 4, two regions are to the region 2 differed on the different wafers, and the formed in a semiconductor crystal wafer base by dopresistance variation with temperature and sensitivity ing boron (B) as an impurity into the surface of the variation with temperature were measured. The results wafer which is sliced from a P-type silicon (Si) semiare explained later. Table I shows the thicknesses of conductor single crystal, and strain gauges of different each sample and the thicknesses of the layers, or retemperature coefficients of resistance are obtained by gions l and 2 together with the measured resistance varying the proportionate volumes, or share ratio, of and sensitivity of each sample:

TABLE 1 Thickness Resist- Sensi- Region Region ance, tlvity 1 (mm.) 2 (mm.) 0 (0 0.) (0 0.) Note 0.194 0.006 149 165 0.094 0.006 224 155 0.044 0.006 300 145 0.044 None 889 185 Crystal wafer ortion. None 0.006 453 125 Portion of boron diffusion layer.

the said two regions. Initially, a P-type (Si) crystal In the above table, the sample D had the same comwafer base, wholly doped with boron, whose resistivity position as that of the base crystal wafer l, or 10. The is 0.46 Q-cm at a temperature of 20 C, is shaped into sample E had only a boron diffusion layer 2. The fora disc of 25mm diameter and 0.42mm thickness by polmer sample was obtained from the wafer by elimiishing with SiC powder of 3 microns diameter, and then nating the region 2, and the latter sample was obtained the disc is etched to reduce the thickness to 0.35mm so by eliminating the region 1 by polishing and etching. In as to eliminate the damaged surface layer ther fl Table l the values of resistance and sensitivity at 0 C After cutting off the edge of said disc, a base wafer 10 are shown, and sensitivity (K) indicates resistance variof rectangular form in section is obtained, as shown in ation caused by the applied strain defined by the fol- FIG. I. lowing formula:

The wafer 10 is next disposed in a geaseous mixture K (R kR of nitrogen and oxygen passed through and over B 0 which is retained at a temperature of 1,200 C and the (where, 6: strain applied to strain gauge, R, and R.

wafer is held at l,200 C for I20 minutes, thereby obresistance of strain gauge when no strain and strain 6 taining a layered wafer 20 with a boron diffusion layer are applied, respectively).

2 on the whole surface thereof without changing the The resistances of the respective samples above were general shape of base wafer 10, as shown in FIG. 2. The measured at temperatures ranging from 0 to 100 C by central region 1 of wafer 20, inside layer 2, retains the intervals of 5 C, and the resistance variations of the resame boron doped composition as that of the base spective samples were calculated according to the forwafer 10. The boron density of the diffusion layer 2 is 50 mula P/P, (where P, resistance at 0 C, P resistance about l l0"/cm and the depth of layer 2 is about 6;;.. at respective temperatures). The results are shown in Next, as shown in FIG. 3, the wafer 30 having theboron FIG. 10 as characteristic curves A 8,, C,, D, and E, diffusion layer 2 only on its upper surface is obtained for the samples A, B, C, D and E, respectively, where by polishing in the same manner as above to eliminate the abscissa indicates temperature (C) and the ordithe layer 2 on the lower surface and all the side surfaces nate indicates the resistance variation (P/P of the wafer 20. Then, the lower surface of the region It is apparent from FIG. 10 that the resistance varial is etched so as to vary the proportionate volumes tion with temperature depends on the share ratio the (share ratio) of the region 1 to the region 2. ratio of the thickness of layers 1 and 2). When layer 2 thickness of layer 2 is proportionately increased with respect to the thickness of layer 1. The values of the rethickness of 600A under a vacuum of l.5Xl mmHg and at a temperature of 250 C. Wafer 50 is heated in a gaseous mixture of nitrogen and oxygen for 60 minutes at i,200 C so as to dope Au into the whole body sistance variation with temperature of the samples A, 5 of the wafer, and as shown in FIG 6 an Au doped layer and C whlch 'helhde both the region h the h wafer 60 is obtained having Au doped region 15 in the gion 2, fall between the values of the variation resiseemer thereof, the region 25 wherein Au is doped into tance with temperature of the sample D which includes a boron diffusion layer, and he remaining Au evaPoonly the regleh 1 e that of the sample E which rated film surface 5'. The diffusion coefficient of Au in eludes only the'regloh the silicon crystal is about 3.5Xi0 cm! per second at The sensitivities of the respective samples A E were 12000 C so in this embodiment Au is doped ahnost "ieasuredoand calfmlated at tenlperamres s e yq uniformly into the silicon crystal wafer of 0.35mm 0 C by 'htervals h 5 and the sehsmv'ty thickness. in the'boron diffusion layer 25, the boron variation calculated according to the formula, K/K, density is ns s the layer depth is about and (where K,: sensitivity at 0 C, K: sensitivity a respecthe Au density is about 1s a tive temperatures). The results are shown in FIG. 11 Next, a wafer, doped with Au as above, and having as eharaetenshe curves and E2 for the a region of boron diffusion only on its upper surface, samples A, i3, C, D and E, respectively, Where t e is obtained by polishing as in the first embodiment, to h e mdleatee and the ordlnate 2e eliminate the remaining Au evaporated film surface 5' mdlqates the sensmvlty e)- on the lower surface of the wafer 60 and the boron dif- It hppareht from 11 h F Sehs'hvty varla fusion layer 25 on the lower and side surfaces of the temperature e e a Shhhar tendency to that wafer 60. The resultant wafer is reduced to a desired of the resistance variation with temperature. The sensithickness by eliminating part f the iower portion f tivityvariation tends to be smaller as the proportionate gieh 15 by etching so as to vary the share ratio of the thickness, or volume, (share ratio) of the region 2 be- 25 region 15 to the region 25. comes larger, and thus the sensitivity variation yalues Using five different wafers having diff t share of the samples A, B and C fall between the variation ties of the region 15 to the region 25, Strain gauges values of the h e D and were made by cutting the wafers to 5mm length and From the above It clear that by partly dopmg 0.4mm width in the same way as in the first embodirhies into? semieohhuetor crystal wafer base to e ment. As a crystal direction connecting the electrodes plural regions of a single conductor type, the regions of the Strain gauges the ll1 was chosen The having different temperature coefficients of resistance, mined Strain gauges were labeled as samples F, G, H. a wafer havhg a different Wi m eoeffieleht of i and J and the resistance variation with temperature resistance than that of the said base can be obtained, and the Sensitivity variation with temperature were whereby a strain gauge of desired, or nearly desired measured as in the first embodiment value of temperature. eoeffieleht can h made" Table 2 below shows the thickness of each sample F Moreover Yarymg the Share who of the reg'ohs J, the thickness of its region 15, and its region 25, as through ehmmahoh of part or most h at leashohe of well as the resistance and sensitivity of each sample at said regions, the temperature coefficient of resistance e C:

' flees; e

Thickness Region Region Resist- Sensi- 15 25 ance Kl tivity (mm) (mm.) (0 (0 0.) Note 0.194 0.006 658 144 0. 004 0.006 700 14a 0.044 0.006 824 143 0.044 None 9, 551 151 Portion of Au doped crystal. None 0. 006 902 142 Portion of Au doped boron diffusion layer.

can be freely varied and, therefore, a strain gauge of in Table 2, sample 1 corresponds to a single region 15 the desired value of the temperature coefficient can be only, formed of P-type silicon crystal wafer doped with made with ease and with accuracy. in the same way, the Au, and the sample J corresponds to a boron diffusion sensitivity variation with temperature of the strain layer 25 only, doped with Au. The former sample was gauge can be made small. obtained from the wafer by eliminating the region 25, EMBODIMENT 2: The second embodiment employs the PP? was obtainfid y eliminating the a method which includes the initial steps of the first emregion y p ng and e ch gbodiment oted bov A l d w f 20 FIG, 2) The resistance variation with temperature of each having a boron diffusion layer 2 on the whole surface sample was ed d Ca cu ated as in the first emis made as in the first embodiment, using a P-type silibodiment, and the results are shown in FIG. 12 as charcon crystal wafer doped with boron see FIG. 1), acteristic curves F,, 6,, H,, i and J for the samples F, Then, as shown in FIG. 5, wafer 50, having boron difful and respectively, Where the abscissa indicate-S sion layer 2 on the upper and side surfaces and having temperature (C) and the ordinate indicates resistance both an Au evaporated film 5 and a boron diffusion variation (PlP layer 2 on the lower surface thereof, is formed by first eliminating the oxide film of the boron diffusion layer 2 on the lower surface of the wafer 20 with HF, and by then evaporating Au onto the lower surface to the it is apparent from FIG. 12 that the temperature coefficients of resistance (slopes of curves F 6,, H of the samples F, G and H, each having both said regions 15 and 25, tend to be negative from zero as the proportionate thickness of the layer 15 to the layer 25 (share ratio) becomes larger. The temperature coefficient of resistance in the sample .I, having only the region 25, is positive, while that of the sample I, having only the region 15, is negative, and the said coefficients of the samples F, G and H having both regions lie between the coefficient of the sample J and the coefficient of the sample I.

The sensitivity variation with temperature of each sample was also measured and calculated as in the first embodiment, and the results are shown in FIG. 13 as characteristic curves F,, G,, H,, l, and J, for the samples F, G, H, l and J, respectively, where the abscissa indicates temperature (C) and the ordinate indicates sensitivity variation (K/K It is apparent from FIG. 13 that the sensitivity variations with temperature show the same tendency as the resistance variation with temperature for the same sample, and the sensitivity variation tends to be smaller as the share ratio of the region 15 to the region 25 becomes larger, sample F and its curve F, showing less change in sensitivity than sample H and its curve H The sensitivity variations for samples F, G and H fall between the corresponding variations for samples I and J.

From the above it will be clear that in the second embodiment, a strain gauge of the desired temperature coefficient of resistance can be obtained as in the first embodiment, and fursther, both positive and negative regions are formed so that the resultant coefficient can be zero, or negative. As previously explained, the two regions are formed by utilizing boron as a first impurity doped into the surface of a semiconductor crystal wafer as a base, the wafer having been sliced from a P-type semiconductor single crystal as in embodiment l, and thereafter Au is doped as a second impurity into the whole body of the wafer, thereby obtaining two regions of a single conductive type and of different temperature coefficients of resistance, positive and negative. Thus, by varying the share ratio of the said two regions, strain gauges of desired coefficient such as positive, negative, and zero can be obtained with ease and accuracy. The above-described sample H and curve H in FIG. 12 shows a temperature coefficient of resistance which is nearly equal to zero in the temperature range between and 100 C, and this characteristic makes it very practicable and suitable for use as a strain gauge. in addition, the sensitivity variation with temperature can be made small in the second embodiment.

DISCUSSION OF THEORY INVOLVED IN THE PREFERRED EMBODIMENTS From the above it will be noted that a semiconductor crystal wafer, having regions of different coefficients of resistance, evidences a temperature coefficient of resistance for the over-all wafer different from that of the respective regions. The theory behind this phenomenon will be explained as follows, first referring to the strain gauge 40 shown in FIG. 4 as being comprised of regions 1 and 2. The first order of approximation of the resistances of the respective regions can be represented as follows:

The resistance of R, of the region 1 is: R, R (l a,t+K e) eq. l-l

the resistance of R of the region 2 is:

and the resultant resistance of the strain gauge 40 is:

where, R R and R are the resistance values of the region 1, the region 2 and the whole piece, respectively, under the conditions of standard temperature and of no strain applied; 01,, a, and a are the resistancetemperature coefficients of the region 1, the region 2 and the whole piece, respectively: K K, and K are the sensitivity values of the region 1, the region 2 and the whole piece, respectively; t is the difference between the measurement temperature and that of the standard; and e is the strain.

And in this example, the resultant resistance R is approximately equal to the resistance of the two regions as if connected in parallel, or

eq. 2-l and since the temperature coefficients of resistance and strain sensitivity are directly dependent on resistance, the following similar equations apply:

Considering eq. 22 when R is very small the term (l /R approaches a, and when R becomes very large the term oq/R approcahes zero. Conversely, when R becomes very large and R very small (IX/R10 approaches zero and az /R approaches 01,. Thus, the temperature coefficient of resistance a tends tohave a value between the coefficients a, and a and the coefficient a can be varied at will by changing the ratio between the resistance R and R (that is, the share ratio of two regions).

The temperature coefficient of resistance of the semiconductor is, if the same atom is used as an impurity, primarily dependent upon the density of the impurity. As in the first embodiment, when the surface of a semiconductor crystal wafer homogeneously doped with an impurity to a shallow level, is further doped with the same impurity, a region of different impurity density, that is, a region having a different temperature coefficient of resistance from that of the undoped wafer is obtained. As apparent from the eq. 22, the said coefficient a can be varied by varying the share ratio between the said two regions (that is, by means of changing the ratio between the resistance values R and R and thus a strain auge of a desired temperature coefficient of resistance can be obtained.

When one of the said coefficients a, and a is positive, and the other negative, the resultant coefficient a can be made negative, or zero as well as positive, by adjusting the ratio between the said resistance values R and R,,,.

The electron energy distribution in a semiconductor crystal may be varied by doping impurities into the crystal, and as a result, the density of the carriers tends to increase as the temperature rises. On the other hand, because the mobility of the carriers decreases as the temperature rises, the temperature coefficient of resistance of the semiconductor crystal becomes negative by doping impurities to a deep level in such manner that the increasing rate of the density of the carriers becomes larger than the decreasing rate of the mobility of the carriers with the rise of temperature. And, therefor, by varying the characteristics as described above, both a region of positive temperature coefficient of resistance and a region of negative coefficient can be formed in one semiconductor crystal wafer. In this case, because diffusion coefficients of deep level impurities in a semiconductor crystal are generally much larger than those of shallow level impurities, they dif- In the second embodiment, the precentage density (X) of Au (X/Y) X I (Y being the carrier density) The density (X) of Au isabout 4.5Xl0, and, therefore, in the said boron diffusion layer (XIY X 100 is about (4.5Xl0 l00)/(4Xl0') or 1.1 percent, the said coefficient in this region being positive, while in the region outside the diffusion layer, (XI Y,) X 100 is about (4.5Xl0 lO0)/(5 l0) or about 90 percent, said coefficient therein being negative.

For convenience of explanation, in the two embodiments detailed above, the strain gauges have been described as being formed of silicon semiconductor crystal with boron (B) and Au as the impurities, but the crystal wafer and the impurities are not limited to those described, and the material shown in Table 3 below, for example, may be employed:

fuse into the whole space of the crystal wafer in a short period of time, and it is difficult to form a diffusion layer in a defined part of the crystal wafer.

It is, however, not difficult for form two regions, one of which has a positive temperature coefficient of resistance and the other a negative coefficient in one semiconductor crystal wafer. For example, this is easily done by adjusting the density rate between the densities of a shallw level impurity (boron, forming the diffusion layer) and a deep level impurity (Au) as in the second embodiment described above.

Taking the second embodiment as an example, the region of the semiconductor which was doped with boron as a shallow level impurity has a positive temperature coefficient of resistance, and also, according to experimental data, when the density of Au as a deep level impurity is doped in said region to a density lower than about l0 percent of the density of the carriers (nearly equal to the density of the boron) before the gold is doped, the temperature coefficient of resistance of said region is changed from that of the region before Au is doped, but said coefficient does not become negative. On the other hand, the density of Au as a deep level impurity doped higher than about 50% of the density of the carriers before Au is doped, causes the temperature coefficient of resistance to become negative. And, therefore, according to experimental data, the diffusion layer region with a positive coefficient (the said density of carriers (y) is: I 4Xl0/cm3) is formed by doping boron into the surface of the crystal wafer (the original density of carriers before doping with boron being: Y, 5 l0'cm), and next, so as to keep positive the said coefficient in the diffusion layer, the density of Au to be doped into this layer in made lower than percent of said density Y and in order to make the said coefficient in the region outside the diffusion layer negative, the density of Au to be doped into this region is adjusted to be more than 50 percent of the said density Y,.

In order to satisfy the required density of Au, the density of Au (X) may be selected between 4Xl0X0.I and 5 l0 0.5, so it is easy todope within the permissible range of about l6 in this way.

It is generally difficult to obtain a strain gauge with a zero, or negative temperature coefficient of resistance using shallow level impurities, but the use of deep level impurities of Table 3 enable the obtainment of zero, or negative coefficient much more easily.

The formation of regions of a single conductive type and of different temperature coefiicients of resistance can be obtained with any combination of crystal and impurities shown in Table 3, and the densities, or the kinds of the impurities of the respective regions may be varied in the manner as set forth above for the two described embodiments.

It is here noted that the positions of the two regions and their shapes in the crystal wafer are not limited to superposed double layers as described in the two embodiments above, but is is possible to obtain a strain gauge having a diffusion layer 27 at the left side of the upper surface, as shown in FIG. 7, by etching away the right side of the upper surface of the wafer shown in FIG. 3. It is also possible to obtain a strain gauge having a diffusion region 28 shown in FIG. 8, on the central part of the upper surface of the wafer by utilizing the conventional selective masking diffusion method. It is further possible to obtain a strain gauge having a diffusion region 29, as shown in FIG. 9, at the left end thereof by diffusing selectively onto the left end of the wafer. While it is most preferable to form two regions only from the viewpoint of varying the share ratio thereof, it is also possible to form three, or more regions in one wafer.

Referring again to FIG. 9, it is pointed out that it is relatively easy to form region 29 by doping with a deep level impurity from the left end and terminating the doped region near the center because of the relatively large length of the wafer. Another deep level, or a shallow level impurity may be doped into region 19 from either the top, or end surface and the share ratio of the two regions may be easily modified by cutting off an end portion of the region 19 before the ohmic lead is connected.

In order to vary the share ratio of the respective regions, polishing and etching are employed as described in the two embodiments, and this treatment may be performed before, or after obtaining the thin strip forming the strain gauge.

The present invention primarily relates to a semiconductor stran gauge and, therefore, it is generally desired that the absolute value of the temperature coeffcient of resistance be small, or zero, but the present invention can be employed to produce a thermistor which requires a large absolute value of temperature coefficient of resistance.

According to the conventional production method of making a strain gauge, a semiconductor crystal is first produced and sliced to a crystal wafer, and then the wafer is cut to the appropriate size for a strain gauge, so that the temperature coefficient of resistance of the strain gauge is equal to that of the semiconductor crystal. And, therefore, to obtain strain gauges of five different coefficients, five kinds of corresponding semiconductor crystals must be produced. By the production method of the present invention, however, a single semiconductor crystal may be produced and wafers of different coefficients can be obtained by applying the inventive production method to wafers cut from the same crystal, so that many strain gauges having different coefficients can be obtained from one semiconductor crystal.

Although certain specific embodiments of the invention have been shown and described, it is obvious that many modifications thereof are possible. The invention, therefore, is not intended to be restricted to the exact showing of the drawings and description thereof, but is considered to include reasonable and obvious equivalents.

What is claimed is:

l. A method of producing a P-type semiconductor strain sensitive element having a temperature coefficient of resistance which is nearly equal to zero, comprising the steps of:

l. Forming a semiconductor crystal of a silicon crystal of P-type uniformly doped with boron,

2. cutting a wafer from said crystal,

3. diffusing boron into the surface of said wafer to form a boron diffusion layer,

4. diffusing gold into said wafer and boron diffusing layer to the amount that the density of gold diffused into said diffusion layer is lower than 10 percent of that of the boron in said diffusion layer and the density of gold diffused into said wafer is higher than 50 percent of that of the boron in said wafer before diffusing with boron in said third step described above so as to form said boron diffusion layer and wafer into two regions having positive and negative temperature coefficients of resistance respectively.

2. A method for producing a P-type semiconductor strain sensitive element according to claim 1, further comprising removing a portion of at least one of said regions to the amount that the resistance variations with temperature of said two regions cancel one another.

* i i i 

2. cutting a wafer from said crystal,
 2. A method for producing a P-type semiconductor strain sensitive element according to claim 1, further comprising removing a portion of at least one of said regions to the amount that the resistance variations with temperature of said two regions cancel one another.
 3. diffusing boron into the surface of said wafer to form a boron diffusion layer,
 4. diffusing gold into said wafer and boron diffusing layer to the amount that the density of gold diffused into said diffusion layer is lower than 10 percent of that of the boron in said diffusion layer and the density of gold diffused into said wafer is higher than 50 percent of that of the boron in said wafer before diffusing with boron In said third step described above so as to form said boron diffusion layer and wafer into two regions having positive and negative temperature coefficients of resistance respectively. 